Cardiac pacemakers are electrical devices that function to regulate the pace of the heartbeat. These vital therapeutic devices are surgically implanted in the patient's body, where they may remain for years. A typical implanted pacemaker operates by furnishing, through an electrical lead attached to the ventricle of the heart, stimulation pulses which the heart is not providing. Generally there are two types of electrode systems used in cardiac pacing. The bipolar system of operation makes use of two electrodes situated near the tip of a lead which is placed into the heart; the electrodes are typically rings which conduct the stimulus pulse and sense natural cardiac signals. A unipolar electrode arrangement makes use of a single electrode placed on the lead, generally near the tip, and another electrode at a location remote from the electrode tip with the return current path being through body tissues. The other electrode is most often a portion of the pacemaker housing; this conveniently provides an electrode of large surface area contacting the body. Alternatively, the other electrode can be on the lead itself proximal to the tip electrode.
The attachment of the heart lead to the pulse generator unit is an important step in pacemaker implantation. Problems with the connection are relatively common, with attendant pacemaker malfunctioning. For many years the connector system for a pacemaker has conventionally consisted of a plastic "connector top" with molded-in terminal blocks containing set screws.
Modern pacemaker leads are coaxial; they are plugged into the terminal receptacles in the pacemaker and are secured by tiny allen head set screws which hold the lead terminals in position and also provide the necessary electrical contacts. Terminal wires extending through hermetic feedthrough elements into the body of the pacemaker are resistance welded to blocks in which the set screws are mounted. The set screws are supposed to be torqued at a level of about 18 inch-ounces.
Drawbacks to the set screw connector system have included: (a) a specially designed wrench is necessary for tightening the set screw; (b) the set screw threads may be stripped due to overtightening; and (c) the set screw must be sealed from body fluids after tightening.
The set screw type of construction presents other problems as well. One is that the set screw sometimes protrudes into the bore of the connector to a point where it blocks the proper insertion of the pacemaker lead. A physician attempting to attach the heart lead to the generating unit may attempt to force the lead terminal into the blocked bore produced by a protruding set screw. Frustration of the physician as well as more serious problems are likely to occur.
Another problem results from overtorquing of the set screw with a lead terminal in position. The set screw that bears against the outer ring of a coaxial lead termination may deform the ring to the point where it is impossible to withdraw the lead terminal from the pacemaker housing. When the time comes for replacement of the pacemaker it may be necessary to actuall cut the leads so that the pacemaker can be removed. In that event not only must the pacemaker be replaced, but also the heart lead as well.
As an alternative to the set screw method of connection, various manufacturers have developed connector systems that use various types of spring contacts. The springs are typically small fingers which contact the heart lead terminals. Success has been limited with such designs primarily because of less-than-satisfactory mechanical contact. Electrical contact is also less than optimal because it is made at specific points around the lead terminal rather than over a large contact surface area.